Crossed-field amplifier



March 1963 E. C. OKRESS 3,082,351

CROSSED-FIELD AMPLIFIER Filed Jan. 6. 1960 c INVENTOR F g- B Ernest C. Okress D ATTORNEY United States Patent 3,082,351 CROSSED-FIELD AMPLIFIER Ernest C. Okress, Elmira, N.Y., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., :1 corporation of Pennsylvania Filed Jan. 6, 1960, Ser. No. 802 Claims. (Cl. 315--39.77)

This invention relates to an electron discharge device and more particularly to a microwave amplifier of the crossed-field type.

Although the efiiciency, over-all weight and compactness assure an increasing role for the reentrant beam backward wave strapped magnetron amplifiers (RB-BWMA), alone or in combination with klystrons and traveling wave tube drivers, certain restrictions have already been encountered with it which limit its applications. These restrictions relate particularly to the feedback problem due to the reentrant beam and backward wave operation. Consequently, a plate modulator is required to stop oscillations after cessation of the input signal.

The approach that has heretofore been taken to the problem of increasing peak power output in the strapped RB-BWMA has been, aside from improving mode stability, that of resorting to a waveguide instead of a coaxial output termination, while retaining the relatively low impedance but fragile and close spaced straps, because of bandwidth consideration, and providing an unbalanced coaxial or parallel wire transmission line coupling between the waveguide terminals and straps.

To alleviate the cited limitations a non-reentrant doubly periodic and closed forward wave structure is proposed with interval attenuation at the back of the delay structure. Thereby feedback is reduced to the extent of not requiring a modulator to stop oscillations on cessation of the input pulse. Furthermore, strapping of the usual symmetric periodic structure and its coupling to the terminations by a coaxial or parallel wire transmission line, as common in the strapped RB-BWMA, are also avoided.

It is, therefore, 'a general object of this invention to provide an improved crossed field microwave amplifier.

It is another object of this invention to provide a microwave amplifier of the crossed-field type having a considerably higher power capacity, a considerably higher frequency limit, and substantially higher gain than that of prevailing strapped RB-BWMA.

It is a further object of this invention to provide a crossed-field amplifier having a non-reentrant doubly periodic and closed structure and coupling chamber or waveguide, of low energy storage characteristics, and normal reentrant interaction space.

These and other objects of this invention will be apparent from the following description taken in accordance with the accompanying drawings throughout which like reference characters indicate like parts, which drawings form a part of this application, and in which:

FIG. 1 is a cross-section view along line I-I of FIG. 2 of a crossed-field amplifier embodying the principle of this invention;

FIG. 2 is a section view FIG. 1;

FIG. 3 is a schematic view of an elementary section of the periodic system shown in FIG. 1;

FIG. 4 is a cross-section view along lines IV-IV of FIG. 1; and

FIG. 5 is a section view along lines VV of FIG. 1.

In FIGS. 1 and 2, there is shown a crossed-field amplifier which comprises an evacuated structure including an electron-emitting cathode 12, a double periodic and closed structure of a plurality of circuit elements 14 and an anode block 16 having a substantially circular opening 13 therein. The circuit elements 14 are arranged to form along the lines II-II of 3,ii82,35i Patented Mar. 19, 1963 an enclosed substantially circular region. The cathode 12 is centrally mounted within the enclosed circular region and separated from the circuit elements 14 by a uniform interaction space 20. Surrounding the circuits 14 is an annular coupling chamber or coaxial waveguide 22v having an input microwave termination 24 and output microwave termination 26, with appropriate direction of axial magnetic field.

The microwave terminations 24 and 26 shown in the drawings comprise a length of hollow rectangular wave guide 34, as can be seen more clearly in FIG. 4 and FIG. 5, across which is mounted a window 36. The window 36 is made of a material that is transparent to the passage of microwave energy, and the window 36 also serves as a vacuum seal. It will be necessary to include a taper or step portions 38, 40' to obtain the desired impedance transformation between the hollow rectangular waveguide 34 and the coupling chamber 22.

The anode block 16 is made of a material having a very high electrical conductivity such as copper. The circuit elements 14 comprise a substantially U-shaped double end closed member having very high electrical conductivity such as copper. The members forming the circuit elements of the double periodic structure which are usually referred to as vanes are closed by end plates 28 and 3G by any suitable method which gives good electrical and thermal conductivity, such as brazing, so that the longitudinally open end of the structure faces the cathode 12. The axial length of the elements 14 may be one half the operating wavelength. The peripherical length of the diode drift section in the interaction space 20, between input and output terminations, is made large enough for space charge debunching to occur in order to reduce feedback to tolerable levels. Simultaneously, this drift length is made short enough to prevent excessive leakage current in order to maintain high efficiency. These conflicting objectives indicate diode drift lengths of several (cg. -5) wave lengths, depending upon gain. The end plates 28 and 30 are substantially flat ring-shaped members which are also formed of material of high electrical conductivity such as copper. The outer diameter of end plates 28 and 30 is substantially the same as the diameter of the opening 18 in the anode block 16. If desired, the vanes 14 and the end plates 28 and 30 may be combined into a subassemhly by the use of a suitable brazing jig. The subassembly is then assembled in proper position within the opening 18 of the anode block 16 by brazing, for example.

The cathode 12 may be provided with a means for heating it to electron emissive temperature by suitable means such as a heater 10. However, with proper voltage between the cold cathode 12 and circuit elements 14, and homogeneous axial magnetic field in the interaction space 20, together with the input RF. signal, the

,- amplifier may be started in operation Without the aid of heater 1% The axial uniform magnetic field is provided through interaction space 20 by pole pieces 3-3 of a magnet (not shown).

When an RF. signal is applied to the input to the amplifier and a suitable axial homogeneous magnetic field and radial electric field are applied in the interaction space 20, between cathode 12 and circuit elements 14-, the electrons that are emitted from the cathode are acted on by the crossed axial D.C. magnetic and radial D.C. electric fields so that the electrons form space charge spokes similar to that obtained in a magnetron oscillator. Interaction occurs at synchronism between the phase velocity of the appropriate wave space h-ormonic of the electromag netic field of the doubly periodic structure, and the rotating space charge in the interaction space 20. The interaction desired here is of the forward wave type. This requires that the small, as well as large circuits are both coupled (slotted) to the common waveguide 22, otherwise if the small circuits are not (as shown) directly coupled (slotted) to the waveguide 22 the backward wave fundamental is used. In the forward wave case the directions of electron motion and power flow in the circuit are the same. By means of this forward wave crossed field interaction, the electrons abstract potential energy from the DC. electric field and lose it to the RF. field, as they are moved from the cathode into higher potential regions toward the anode, by the RF. field. As a consequence, electrons remain in synchronism with the circuit wave as this conversion of energy takes place. So, the input electromagnetic waves are amplified and increased in amplitude as they propagate along the circuit toward the output termination. There is a phase shift per circuit element in the direction of power flow in the interaction space. The phase shift across the circuit will be zero and its characteristic impedance infinite at the lower cutoff frequency of the circuit, and 1r radians and Zero ohms, respectively, at the upper cutoff frequency. There remains a substantial range in which the phase shift is nearly linear with frequency.

FIG. 3 shows the basic structure of the non-reentrant periodic system used in this amplifier. It is essentially an asymmetric, doubly periodic and closed system. That is, region defined by F, G, H, l, i, K of the waveguide, together with region defined by E, F, K, L of the periodic structure, comprise one circuit of the doubly periodic structure, and region defined A, B, C, D comprises the other circuit. The boundary conditions at the septum G, H; I, 1; etc., are such that the region F, G, H, I, J, K also serves as a portion of the waveguide between the input and output terminations.

The use of this system eliminates the need for strapping of the periodic structure and coupling of the straps to the terminations by a coaxial or parallel wire transmission line. The absence of straps in the periodic system of the invention also permits considerable increase in the high frequency operational limit. The peak power handling capabilities are likewise considerably improved since a higher radio frequency voltage can be utilized without breakdown. Furthermore, the closing end covers increase the thermal dissipation capacity of the vanes considerably so a substantially greater average power handling capacity results.

In its broader aspects, this invention reveals a structure for a crossed-field -amplifier possessing unique features sought but not heretofore available with strapped RB- BWMAs. These features include substantial improvements in frequency limit as well as power and gain, which are not limited by strapping considerations. To achieve these features, an asymmetrical system is proposed in a manner which enables the elimination of straps and the associated transmission coupling lines to the terminations. This invention also permits a substantially greater number of circuits to be used in the periodic structure without affecting the stability due to its superior control of the loading and distribution of RF. field components. The upper limit in the number of permissible circuits, although much higher than for strapped RB-BWMAs, is determined nevertheless by other factors common to all reentrant beam crossed field tubes (e.g. adjacent RF. field wave component feedback).

The relatively large cathode possible with large number of circuits is not a major concern at low power levels but becomes a very important advantage at very high frequencies and high power levels since it provides a greater surface area for electron emission and effective heat dissipation.

Due to the closure of the axial ends of the slow wave structure, coaxial and septate modes must be considered since they are determined in part by the axial vane length. These modes involve the cathode, the small circuits, and the periphery of the coupling chamber. The coaxial and septate modes behave like the principal transverse electric modes and like these modes are a potential source of operational instability though they are controllable by appropriately positioned attenuation and by means previously mentioned.

For reasons previously cited the gain of the amplifier of this invention is not so limited as in the strapped RBBWMA. The gain however is a linear function of the number of sections of the circuit. Relieving substantially the restriction on the number of sections which can be used with respect to mode competition also allows a more favorable magnet weight and circuit impedance for better matching of the network to its terminations. The construction of the proposed periodic structure is more rugged and better heat dissipation is realizable than with prevailing strapped RB-BWMA since the covers totally enclose the axial vane ends of the doubly periodic structure and waveguide. Although these covers are applied in part for controlling the coaxial mode spectrum of the periodic structure, they automatically provide highly desired and important thermal contacts to the vanes of the resonator, thereby substantially increasing their power dissipation capacity. Furthermore, these end covers substantially increase the rigidity of the vanes and reduce the tendency to sparking between otherwise exposed vane tips and cathode hats. These end covers also provide substantially greater ease of fabrication of the device and facilitate the accurate alignment of the vanes. If desired, the vanes may be formed so as to optimize the circuit impedance.

While the present invention has been shown in one form only, it will be obvious to those skilled in the art that it is not so limited but is susceptible of various changes and modifications without departing from the spirit and scope thereof.

I claim as my invention:

1. An electron device for amplifying electromagnetic wave energy comprising an evacuated structure including a plurality of electrical circuits comprising substantially U-shaped members of electrically conducting material provided therein and spaced to form an enclosed reentrant interaction region, said circuits totally closed at each end, a cathode electrode providing space charge within said enclosed reentrant interaction region, means to provide a uniform axial magnetic field throughout said reentrant interaction region, a non-reentrant waveguide for propagating electromagnetic waves in the space between the evacuated structure and said plurality of circuits, whereby said electromagnetic wave is amplified due to interaction between the electromagnetic field of the circult and the space charge in the reentrant interaction region.

2. A crossed-field amplifier comprising an evacuated structure and having an input termination and an output termination for electromagnetic waves, said termination connected to an annular coupling chamber extending through said evacuated structure, the outer periphery of said annular coupling chamber defined by a conductive shell member, the inner periphery of said annular cou pling chamber defined by a plurality of circuits comprising substantially U-shaped members of electrically conducting material, said circuits equally spaced, said circuits being closed at their ends, a cathode electrode within said interaction space, means to provide a uniform axial magnetic field through said interaction space, means for providing electron motion within said interaction space, whereby an electromagnetic wave is amplified due to a transfer of energy from said electrons to said electromagnetic wave as the radio electromagnetic wave is propagated around said coupling chamber.

3. A crossed-field amplifier comprising an evacuated structure and having a radio-frequency transmission line extending through said evacuated structure, said transmission line having input and output terminations and an ductive shell member, said inner wall formed by a plutrality of resonators of generally U-shaped configuration, said resonators spaced on a circle and separated by equal gaps, said resonators enclosing an interaction space, said resonators being closed at each end, a cathode electrode within said interaction space, means for providing a uniform axial magnetic field through said interaction space, means for providing electron motion within said interaction space whereby said electromagnetic waves gain energy due to interaction with said electrons through said gaps between said resonators.

4. A microwave amplifier comprising an evacuated structure including a plurality of circuits forming a substantially circular space, said circuits comprising members of electrically conductive material having their axes substantially parallel, each of said circuits being separated by a gap, end plates provided at each end of said circuits, an electron emitting cathode within said circular space, means to provide a uniform axial magnetic field through said space, means to provide a radial electric field between said cathode and said circuits whereby the electrons emitted by the cathode form a rotating space charge within said circular space in a direction dependent upon the magnet polarity, a waveguide structure for propagating electromagnetic waves around the periphery of said plurality of circuits whereby said electromagnetic waves gain energy by interaction, through said gaps between said circuits, with said space charge of electrons,

and an output termination on said waveguide for utilizing said amplified waves.

5. A crossed-field amplifier comprising an evacuated structure including a multi-section asymmetrical periodic structure including a plurality of electrically conducting members having their axes substantially parallel, each section of said periodic structure having a small circuit and a large circuit, end plates provided at each end of said conducting members, a part of each said large circuits also forming a part of a waveguide transmission line extending through said evacuated structure, said small circuits forming a substantially circular space, said waveguide transmission line extending around the periphery of said circular array of said small circuits for propagating electromagnetic waves, means for forming a space charge of electrons within said circular space whereby said electromagnetic waves are amplified due to energy transferred from said space charge of electrons.

References Cited in the file of this patent UNITED STATES PATENTS 2,673,306 Brown Mar. 23, 1954 2,687,777 Warnecke et al Aug. 31, 1954 2,695,929 Reverdin Nov. 1954 2,815,469 Sixsmith Dec. 3, 1957 2,819,449 Herold Jan. 7, 1958 2,956,204 Dohler et .al. Oct. 11, 1960 

1. AN ELECTRON DEVICE FOR AMPLIFYING ELECTROMAGNETIC WAVE ENERGY COMPRISING AN EVACUATED STRUCTURE INCLUDING A PLURALITY OF ELECTRICAL CIRCUITS COMPRISING SUBSTANTIALLY U-SHAPED MEMBERS OF ELECTRICALLY CONDUCTING MATERIAL PROVIDED THEREIN AND SPACED TO FORM AN ENCLOSED REENTRANT INTERACTION REGION, SAID CIRCUITS TOTALLY CLOSED AT EACH END, A CATHODE ELECTRODE PROVIDING SPACE CHARGE WITHIN SAID ENCLOSED REENTRANT INTERACTION REGION, MEANS TO PROVIDE A UNIFORM AXIAL MAGNETIC FIELD THROUGHOUT SAID REENTRANT INTERACTION REGION, A NON-REENTRANT WAVEGUIDE FOR PROPAGATING ELECTROMAGNETIC WAVES IN THE SPACE BETWEEN THE EVACUATED STRUCTURE AND SAID PLURALITY OF CIRCUITS, WHEREBY SAID ELECTROMAGNETIC WAVE IS AMPLIFIED DUE TO INTERACTION BETWEEN THE ELECTROMAGNETIC FIELD OF THE CIRCUIT AND THE SPACE CHARGE IN THE REENTRANT INTERACTION REGION. 